Because a compound semiconductor crystal comprising the 12 (2B) group element and the 16 (6B) group element in the periodic table (hereinafter referred to as a II-VI group compound semiconductor) has various forbidden band widths, the crystal has various optical characteristics. Further, the II-VI group compound semiconductor has been expected to be used as material for a light modulation element for example. However, in case of the II-VI group compound semiconductor, because it is difficult to control the stoichiometric composition (stoichiometry), it is difficult to grow a good bulk crystal by the traditional manufacture technique.
For example, in ZnTe, a composition at the melting point is shifted to Te from the stoichiometric composition. Thus, in grown crystals, there is some possibility that deposits that are presumably caused by excessive Te, remain. Because the Te deposits have sizes of a few μm and densities of about 105 cm−3, the Te deposits are a cause of a remarkable decrease in the light transmission of a ZnTe single crystal substrate. The ZnTe single crystal substrate having such a low light transmission is not suitable for a light modulation element or the like using an electro-optic effect, in which laser light passes through a crystal having a thickness of about 10 mm.
As a technique for reducing the deposits in the ZnTe single crystal, there is a method for growing the ZnTe single crystal by using epitaxial growth technique. According to this technique, it is possible to manufacture a ZnTe single crystal having a superior crystallinity.
The present applicant has suggested a method for manufacturing a II-VI group compound semiconductor single crystal that comprises at least: a first step of increasing a temperature of a II-VI group compound semiconductor single crystal to a first heat treatment temperature T1 and retaining the temperature of the crystal for a predetermined time; and a second step of gradually reducing the temperature of the crystal from the first heat treatment temperature T1 to a second heat treatment temperature T2 lower than the heat treatment temperature T1 with a predetermined rate (Patent Publication 1). According to the invention disclosed in Patent Publication 1, in the first step, it is possible to eliminate the deposits consisting of the 16 group element (e.g., Te) and in the second step, it is possible to eliminate the deposits consisting of polycrystal and the like.    Patent Publication 1: Japanese Laid-Open Publication No. 2004-158731